An acceleration transducer is a force transducer with an attached so called seismic mass. The inertia of the seismic mass produces a force when being accelerated and this force is measured by the acceleration transducer. The force is the measurand. As the seismic mass is constant, the measured force is proportional to the acceleration. Acceleration is defined as the time rate of change in velocity.
Piezoelectric transducers are comprised of piezoelectric material that creates electric charge when subjected to a force. The electric charge is proportional to the applied force and can be measured directly by means of electrodes. Piezoelectric transducers offer the advantages of a fast response time of a few μsecs and a wide measurand range of the order of 100 Db. Piezoelectric transducers are thus utilized to measure dynamic oscillations and vibrations on machines and structures.
U.S. Pat. No. 3,673,442A discloses a piezoelectric transducer. The piezoelectric transducer uses a quartz crystal as piezoelectric material. It comprises five quartz crystal discs, each of the quartz crystal discs is arranged between two electrodes, forming thus a stack of crystal discs and electrodes. The force to be measured acts along a support axis of the piezoelectric transducer. A clamping sleeve joins the stack and a seismic mass along a longitudinal axis towards a base plate. By doing so, the piezoelectric transducer has a high rigidity along the longitudinal axis and the quartz crystal discs are in close mechanical contact with the electrodes. A close mechanical contact minimizes or eliminates any micro-gaps between the quartz crystal discs and the electrodes and thus assures a clearly defined capacitance of the piezoelectric transducer.
One drawback of quartz crystal is the relatively low piezoelectric sensitivity, especially in comparison with piezoelectric ceramics as piezoelectric material. Piezoelectric sensitivity is defined as the ratio of change in piezoelectric transducer output to a change in the value of the measurand. The change in piezoelectric transducer output is determined by piezoelectric charge coefficients of the piezoelectric material. The greater the piezoelectric charge coefficient, the stronger the piezoelectric transducer output for a given value of the measurand. Piezoelectric ceramics like a mixture of PbZrO3 and PbTiO3, also designated as lead-zirconite-titanate mixed ceramics (PZT) exhibit piezoelectric charge coefficients up to hundred times higher than those of quartz crystal. There is a demand for a piezoelectric transducer with a high piezoelectric sensitivity.
PZT, however, is strongly temperature-dependent and shows an elevated thermal sensitivity shift. The thermal sensitivity shift is defined as the change in piezoelectric sensitivity from within the limits of the operating temperature range of the piezoelectric material referenced to the piezoelectric sensitivity at +20° C. The thermal sensitivity shift is expressed in percent of the piezoelectric sensitivity referenced to the piezoelectric sensitivity at +20° C. The operating temperature range commonly covers −54° C. to +260° C. PZT has a high thermal sensitivity shift of the order of some percent, while quartz crystal has a ten times lower thermal sensitivity shift. The thermal sensitivity shift is a non-linearity in the piezoelectric transducer output, which means that the piezoelectric sensitivity is several percent higher at the higher end of the operating temperature range than at the lower end of the operating temperature range. During a force measurement, the temperature of the piezoelectric transducer is normally unknown, so that the non-linearity over temperature of PZT is a severe drawback of the use of PZT in piezoelectric transducers. This non-linearity over temperature in the piezoelectric transducer output can be determined experimentally by using special calibration equipment. Calibration of a piezoelectric transducer is, however, time consuming and the calibration equipment is expensive. In order to obtain a high linearity in the piezoelectric transducer output over temperature, a low thermal sensitivity shift is desirable.